Error amplifier for switching regulator

ABSTRACT

A differential amplifier compares the voltage from a switching regulator with a constant voltage and provides an output current having a value which is determined by the voltage from the switching regulator. A feedback circuit limits the rate at which the output current increases when the voltage from the switching regulator drops suddenly. Another circuit disables the amplifier and provides a fixed value of output current when the switching regulator is initially turned on.

United States Patent Nowell 3,707,684 Dec. 26, 1972 ERROR AMPLIFIER FORSWITCHING REGULATOR [72] Inventor: John R. Nowell, Phoenix, Ariz.

[73] Assignee: Honeywell Information Systems,

Inc., Waltham, Mass.

[22] Filed: Nov. 22, 1971 [21] Appl. No.: 200,675

[52] U.S. Cl. ..330/9, 330/20, 330/40, 330/24 [5 1] Int. Cl .1103! 1/02[58] Field of Search ..330/20, 40, 9 C

[56] References Cited UNITED STATES PATENTS 2,866,018 12/1958 Bell..330/9 3,678,402 7/1972 Tempe] ..330/9 Primary Examiner-Nathan KaufmanAttorney-Lloyd B. Guernsey et al. and Edward W. Hughes [57] ABSTRACT Adifferential amplifier compares the voltage from a switching regulatorwith a constant voltage and provides an output current having a valuewhich is determined by the voltage from the switching regulator. Afeedback circuit limits the rate at which the output current increaseswhen the voltage from the switching regulator drops suddenly. Anothercircuit disables the amplifier and provides a fixed value of outputcurrent when the switching regulator is initially turned on.

8 Claims, 4 Drawing Figures ERROR AMPLIFIER FOR SWITCHING REGULATORBACKGROUND OF THE INVENTION This invention relates to an error amplifierfor sensing the voltage at the output terminal of a switching regulatorand for providing a current having a value which is determined by thevalue of the voltage from the regulator. This invention relates moreparticularly to an error amplifier which uses a feedback circuit toprotect the switching regulator by limiting the rate of increase ofoutput current when the voltage from the regulator drops suddenly. Theamplifier uses another circuit to protect the regulator on start-up byproviding a fixed value of output current when the switching regulatoris initially turned on.

In high speed data processing systems switching regulators may be usedto provide D.C. power to electronic circuits in the system. Theseregulators are smaller and more efficient than prior art power suppliesso that the regulators may be located in the cabinets which contain thecircuits rather than in a separate cabinet as required when prior artpower supplies are used. Location of regulators near the circuitsgreatly reduces the length of cables which distribute the current to thecircuits and reduces the amount of error signals which may be caused byvariation in voltage in long cables.

The switching regulator may employ a transformer, a pair of siliconcontrolled rectifiers and a source of signals to convert an unregulatedD.C. voltage, such as 150 volts, to an accurately regulated voltage,such as 5 volts. The silicon controlled rectifiers are employed asswitches between the source of unregulated D.C. voltage and thetransformer. The silicon controlled rectifiers are located on thehigh-voltage side of the transformer where the current and power lossesin these rectifiers are low, thereby causing the switching regulator tohave a high degree of efficiency. The regulated D.C. voltage obtainedfrom a secondary winding on a transformer is supplied to a pair ofvoltage output terminals. The transformer provides isolation between theregulated D.C. voltage and the source of unregulated D.C. voltage sothat a short circuit in a silicon controlled rectifier will not causedamage to the microcircuit modules which provide the load on theswitching regulator.

The silicon controlled rectifier is a semi-conductor device having ananode, a cathode and a gate. The silicon controlled rectifier can beused as an ON-OFF switch which can be turned on in a very fewmicroseconds. Normally, the silicon controlled rectifier cannot conductcurrent between anode and cathode thereof until a pulse of currentlarger than a threshold value flows from gate to cathode. If a positivevoltage difference exists between the anode and the cathode when a pulseof current flows into the gate, the silicon controlled rectifier fires;i.e., is rendered conductive and a current will flow from the anode tothe cathode. The rate at which the current flow from anode to cathodeincreases when the silicon controlled rectifier fires must be limited toprevent damage to the rectifier. Once anode-cathode flow commences, thegate has no further control over such current flow. Current flow fromanode to cathode in a rectifier can be terminated only by reducing theanode to cathode current below a holding or minimum current value. Amore detailed description of the operation of a silicon controlledrectifier can be found in the Silicon Controlled Rectifier Manual, 4thedition, 1967, published by the General Electric Company, Syracuse, NewYork.

An error amplifier is coupled to the voltage output terminals of theswitching regulator and develops an output current whose value isdetermined by the value of the voltage at the voltage output terminals.The current from the error amplifier is applied to a rate generatorwhich develops trigger signals whose frequency is determined by thevalue of the current from the error amplifier. The trigger signals arecoupled to the silicon controlled rectifiers in the switching regulatorand cause these rectifiers to deliver energy through the transformer tooutput filter capacitors which are connected to the voltage outputterminal. The error amplifier and the rate generator sense any change inthe value of the regulated output voltage and cause a change in thefrequency of the trigger signals delivered to the switching regulator.

Prior art error amplifiers provide a current having a value which isdetermined by the value of the voltage at the output terminal of theswitching regulator. This current causes a rate generator to developtrigger signals having a frequency which is determined by the currentfrom the error amplifier. These trigger signals are applied to the gatesof the silicon controlled rectifiers in the switching regulators. Whenthe voltage at the output terminals of the switching regulator decreasesrapidly the current provided by the prior art error amplifiers increasesrapidly thereby causing the frequency of the trigger pulses from therate generator to increase to a relatively high value. If the frequencyof the rate generator pulses is too high one of the pulses from the rategenerator may cause a silicon controlled rectifier to be renderedconductive before another one of the silicon controlled rectifiers isrendered nonconductive. When two of the silicon controlled rectifiers inthe switching regulator are conductive at the same time an excessivelyhigh current may flow through these silicon controlled rectifiersthereby causing damage to the rectifiers. What is needed is acombination of an error amplifier and a rate generator which will notallow more than one of the silicon controlled rectifiers to be renderedconductive at the same time.

The present invention provides an error amplifier having a feedbackcircuit which limits the maximum amount of current provided by theamplifier when the voltage at the output terminal of the switchingregulator decreases rapidly. The present invention also provides acircuit which limits the current from the error amplifier when the poweris initially applied to the switching regulator. The limited value ofcurrent from the error amplifier produces a limit on the upper frequencyof the rate generator and prevents damage to rectifiers in the switchingregulator.

It is, therefore, an object of this invention to provide an erroramplifier which provides an output current having a value which isdetermined by the value of the voltage from a power supply.

Another object of this invention is to provide an error amplifier whichproduces a limited value of output current when the power supply isinitially energized.

A further object of this invention is to provide an error amplifierwhich produces a limited value of output current when a low impedanceload is connected to the power supply.

Still another object of this invention is to provide an error amplifierwhich produces an output current having a value which changesrelatively-slowly when the value of the power supply voltage changessuddenly.

SUMMARY OF THE INVENTION The foregoing objects are achieved in thepresent invention by providing a new and improved error amplifier whichdevelops a current having a value which is determined by the voltagefrom the output terminals of the switching regulator. A feedback circuitin the error amplifier prevents a sudden decrease in the voltage of theswitching regulator from causing a sudden increase in the output currentfrom the error amplifier. Another circuit of the error amplifierprovides a limited value of output current when the switching regulatoris initially energized and also provides a limited value of current whena low impedance load is connected to the switching regulator.

Other objects and advantages of this invention will become apparent fromthe following description when taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of theswitching regulator and its associated control circuits including thepresent invention,

FIG. 2 is a schematic drawing of an embodiment of the present invention;

FIG. 3 illustrates a magnetization curve which is useful in explainingthe operation of the circuits shown in FIG. 1;

FIG. 4 illustrates waveforms which are useful in explaining the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly tothe drawings by the characters of reference, FIG. 1 discloses a powersupply system which is designed to provide a constant supply of D.C.output voltage for a wide range of values of output current and formonitoring the current delivered to a load which may be connected to thesystem. As indicated in FIG. 1, the system comprises a switchingregulator 10, a switching regulator control circuit 11 for providingtrigger signals to switching regulator 10, and a circuit 12 formonitoring the current and the voltage delivered by the power supply.The switching regulator control circuit 11 comprises a trigger generator14, a rate generator 15, a recovery disable circuit 16 and an erroramplifier 17. The error amplifier l7 detects any change in voltage atthe output terminals of the switching regulator and provides a signalwhose value is determined by the change in the output voltage. Thesignal from the error amplifier 17 causes the rate generator to developpulses having a frequency which is determined by the value of the signalfrom the amplifier 17. Pulses from the rate generator cause the triggergenerator 14 to develop trigger pulses for the switching regulator. Therecovery disable circuit 15 senses the time that output current is beingdelivered by the switching regulator to the output filter capacitors andprevents the rate generator from delivering pulses during the time thatthe current is being delivered.

The over-current detector 20, the over-voltage detector 21 and theunder-voltage detector 22 sense any abnormal values of current orvoltage at the output of the switching regulator and provide signals tothe fault shutdown circuit 19. When the fault shutdown circuit 19receives a signal from any of the detectors 20, 21 and 22 it provides asignal to the rate generator which disables the rate generator andprevents any pulses from being supplied to trigger the switchingregulator. Switching Regulator As indicated in FIG. 1, switchingregulator 11 includes a pair of transformers 25 and 26, each having aprimary winding and a secondary winding. The primary windings 28 and 30are connected in series and are coupled to the high voltage unregulatedD.C. power supply having a positive output terminal 36 and a negativeoutput terminal 37. A pair of silicon controlled rectifiers 33 and 34control the current supplied by the power supply to the primary windingsof transformers 25 and 26. The anode of silicon controlled rectifier 33is connected to the positive terminal 36 of the unregulated D.C. powersupply and the cathode of silicon controlled rectifier 33 is connectedto the upper end of primary winding 28.'The gate of silicon controlledrectifier 33 is connected to one lead of the trigger generator 14 whichprovides trigger signals to render rectifier 33 conductive. The anode ofsilicon controlled rectifier 34 is connected to the lower end of primarywinding 30 and the cathode of silicon controlled rectifier 34 isconnected to the negative terminal of the unregulated D.C. power supply.A second lead from the trigger generator 14 is connected to the gate ofsilicon controlled rectifier 34 to provide trigger signals to renderrectifier 34 conductive.

The magnetic core employed in transformers 25 and 26 produces themagnetization characteristics illustrated in the magnetization curve ofFIG. 3. The mag netizing force H is equal to the product of the numberof turns in a winding on the transformer core and the number of amperesof current for each turn of wire divided by the length of the core.Since the physical length of the particular transformer core is constantthe magnetizing force of the'transformer is often expressed as thenumber of amperes times the number of turns, or ampere turns." The fluxdensity B is the number of lines of flux per square centimeter of thetransformer core and is determined by the value of the magnetizing forceand the type of material used in the core. A discussion of themagnetization curves can be found in the text book Magnetic Circuits andTransformers by E. E. Staff, M.I.T., 1943, published by John Wiley &Sons, New York, New York.

The operation of the circuit of FIG. 1 will now be discussed inconnection with the magnetization curve shown in FIG. 3 and thewaveforms shown in FIG. 4.

A pair of capacitors 40 and 41 provide predetermined quantities ofelectrical energy to the transformers 25 and 26 each time one of thesilicon control rectifiers 33 and 34 is rendered conductive. Each timeone of the silicon controlled rectifiers 33 and 34 is renderednonconductive the same predetermined quantity of energy is delivered byone of the transformers 25 and 26 through diodes 43 and 44 to a filtercapacitor 48. Prior to the time t, shown in FIG. 4, capacitor 40 of FIG.1 is charged to the polarity shown in FIG. I. At time t a pulse fromtrigger generator 14 renders silicon control rectifier 33 conductive sothat the voltage across the capacitor 40 is supplied to the primarywinding 28 of transformer 25 causing a current I to flow from the upperplate of capacitor 40 through to anode to cathode of rectifier 33,through the primary winding 28 to the lower plate of capacitor 40. Thecurrent I through primary winding 28 causes a change of flux in thetransformer core and causes the operating point to move from point Atoward point C of the magnetization curve in FIG. 3. This change in fluxproduces a voltage across primary winding 28, which limits the rate ofincrease in current through silicon controlled rectifier 33, thuspreventing possible damage to rectifier 33. A positive voltage appliedto the upper end of primary winding 28 causes the operating point tomove upward from point C toward point D. The distance between point Cand point D is proportional to the product of the voltage applied toprimary winding 28 and the duration of time this voltage is applied.

The voltage applied to the primary winding 28 is magnetically coupledthrough the transformer core to the secondary winding 29. Between time tand time secondary winding 29 has a positive polarity of voltage at thelower end of the winding and a negative polarity of voltage at the upperend of the winding. At this time, the voltage across the secondarywinding 29 causes diode 43 to be back biased so that no current flowsthrough the diode or through the secondary winding 28. Capacitor 40provides current I, until this capacitor has discharged at time t, asshown in waveform I of FIG. 4. The area M under the curve of waveform E,(FIG. 4) between time t, and t, is the sum of the products of thevoltage applied to primary winding 28 and the duration of the time thevoltage is applied and this area M represents the total energy stored inthe core of transformer 25. When the voltage applied to primary winding28 has a zero value at time t the operating point reaches point D.

At time t the energy stored in the core of transformer 25 reverses thepolarity of voltage across each of the transformer windings so thatnegative polarity of voltage is developed at the upper end of primarywinding 28. This negative polarity of voltage at the upper end ofprimary winding 28 causes the operating point in FIG. 3 to move frompoint D toward point E and to begin moving toward point A. Again thedistance between point E and point A is proportional to the product ofthe voltage across primary winding 28 and the duration of time thisvoltage is applied. The area N under the curve of waveform E betweentimes and t, is the sum of the products of voltage across primarywinding 28 and the time this voltage is applied. In this area Nrepresents a total energy which the core of transformer 28 returnsthrough the transformer. The voltage across primary winding 28 causescurrent I, to

' charge capacitor 40 to a polarity opposite to the polarity shown inFIG. 1.

The energy in the core of transformer 25 causes the voltage acrosssecondary winding 29 to increase to a value larger than the voltageacross filter capacitor 48 so that a current 1 flows through diode 43 tocharge capacitor 48. The energy which is stored in the core of thetransformer 25 when silicon controlled rectifier 33 conducts isproportional to the difference between the flux at point A and point Don the magnetization curve of FIG. 3; and the energy which istransferred to the secondary winding 29 when silicon controlledrectifier 33 is rendered nonconductive, is proportional to thedifference between the flux at point E and point A.

Since the distance between point A through point C to point D shown inFIG. 3 is substantially the same as the distance between points Bthrough point F to point A, substantially all of the energy which wasstored in the core of the transformer between times t and t is returnedand is stored in capacitors 48 and 49. Capacitor 40 deliverssubstantially the same amount of energy to the transformer each time thesilicon controlled rectifier 33 is rendered conductive so that theamount of energy delivered to filter capacitors 48 and 49 and thevoltage across these capacitors is determined by the frequency of thesignals applied to the gate of rectifier 33 Capacitor 41 also provides apredetermined quantity of energy to the transformer 26 each time siliconcontrolled rectifier 34 is rendered conductive. A more detaileddescription of the operation of the switching regulator can be found inthe US. Pat. No. 3,518,526 by Luther L. Genuit, issued June 30, 1970,entitled Switching Regulator.

Prior to time 22 capacitor 41 is charged to the polarity shown inFIG. 1. At time t a pulse from the trigger generator 14 renders siliconcontrolled rectifier 34 conductive so that current 1 flows from theupper plate of capacitor 41 through the primary winding 30, from anodeto cathode of rectifier 34 to the lower plate of capacitor 41. Current 1through the primary winding and the voltages impressed across thiswinding cause the operating point of the characteristic curve in FIG. 3to move from point A through point C to point D and causing apredetermined quantity of energy to be stored in the core of transformer26. When silicon controlled rectifier 34 is rendered nonconductive, thisenergy is transferred through the secondary winding 31 causing a currentI to charge capacitor 48 as described above.

The amount of voltage across the capacitors 48 and 49 can be controlledby controlling the frequency of the trigger signals which triggergenerator 14 applies to the gates of silicon controlled rectifiers 33and 34. The frequency of the trigger signals is determined by the valueof the current applied to the rate generator 15. When an increase in theamount of current drawn by a load (not shown) connected across theoutput terminals 51 and 52 in FIG. 1 causes the value of the outputvoltage to fall below a predetermined reference level, the frequency ofthe signals from trigger generator 14 increases. This increase in thefrequency of the output signals causes an increase in the rate of energydelivered to filter capacitors 48 and 49 and increases the voltage atthe output terminal 51 of the power supply controls the frequency of thesignal from the trigger generator 14 so that the voltage at the outputterminals 51 and 52 is substantially constant even when the currentdrawn from this power supply varies over a wide range of values.

l060l2 02l0 Error Amplifier As indicated in FIG. 2, the error amplifiercomprises a constant voltage source 54, a difference amplifier 55, anintermediate amplifier 56. a current amplifier 57, a comparator 58, anda minimum current circuit 59. Amplifier 55 includes a differentialamplifier 73 having first and second input leadsand an output lead. Thevoltage received at the first input lead 71 is compared with the voltagereceived at the second input lead 72 and the differential amplifierproduces a voltage at output lead 74 which is determined by thedifference between the voltages at the first and second input leads.When the voltage at the first input lead'7l increases or becomes morepositive the voltage at the output lead 74 decreases. The voltage onoutput lead 74 can be adjusted by controlling the value of the voltageon input lead 72. The voltage on lead 72 can be controlled by thesetting of the potentiometer 64 in the constant voltage source 54. Thevalue of the voltage on output lead,

74 of-the differential amplifier determines the conductivity oftransistors 77, 78 and 87 and determines the value of current to theoutput lead 109 of the error amplifier.

Tenninal 67 of amplifier 55 is connected to the voltage output terminal51 .(FIG. 1) of the switching regulator. Resistors 68 and 70 comprise avoltage divider network which determines the value of the voltage atjunction point 69 and on the input lead 71 of amplifier 73. In atypical'circuit the values of resistors 68 and 70 are chosen so that avoltage V,,, from the switching regulator, of volts provide a voltage of4.5 volts at junction point 69.

When the value of voltage on input lead 71 of amplifier 73 approaches avalue of 4.5 volts the voltage on the output lead 74 causes a current Ito flow from the base to emitter of transistor 77 from the base toemitter of transistor 78, through zener diode 84 and resistor 85 toground. Current I renders transistor 77 conductive so that a current Iflows from terminal 79 through collector to emitter of transistor 77from base to emitter of transistor 78, through zener diode 84 andresistor 85 to ground. Currents I, and I from base to emitter oftransistor 78 render transistor 78 conductive so that a larger currentI, flows from terminal 80 through resistor 81, from collector to emitterof transistor 78, through zener diode 84 and resistor 85 to ground.Current I, through resistor 81 provides a voltage drop of the polarityshown across resistor 81 so that the voltage at the collector oftransistor 78 decreases. The lower value of voltage at the collector oftransistor 78 causes a current I to flow from terminal 88 throughresistor 89, from emitter to base of transistor 87 through transistor78, zener diode 84 and resistor 85 to ground. Current I, renderstransistor 87 conductive so that a current I flows from terminal 88through resistor 89 from emitter to collector of transistor 87 to outputlead 109 and to the rate generator shown in FIG. 1. Current I to therate generator causes the rate generator to develop pulses for thetrigger generator 14 (FIG. 1) and causes the output voltage from theswitching regulator of FIG. 1 to remain at approximately 5 volts. I

When the voltage V, of the switching regulator decreases the voltage oninput lead 71 of the differential amplifier 73 decreases so that thevoltage at the output lead 74 increases. An increase in voltage onoutput lead 71 causes current I, through transistor 77 to increase sothat the conductivity of transistor 77 increases thereby causing currentI, to increase. When current I; increases this causes an increasein thecom ductivity of transistor 78 so that the current I, through transistor78 increases thereby causing an increase in the voltage drop acrossresistor 81 and decreasing the voltage at the collector of transistor78. A decrease in the voltage at the collector of transistor 78 causesan increase in the emitter to base current I of transistor 87 therebyincreasing the conductivity of transistor 87 so that current l to therate generator increases. When the current I to the rate generatorincreases the frequency of the pulses supplied by rate generator 15 andtrigger generator 14 increases. This increase in the frequency of thetrigger signals from trigger generator 14 causes an increase in the rateof the energy delivered to the output filter capacitors 48 and 49 of theswitching regulator and causes the voltage at the output terminals 51and 52 to return to the predetermined reference level.

In prior art error amplifiers a sudden decrease in the voltage V,, fromthe switching regulator could cause the voltage at the output lead 74 ofthe differential amplifier 73 to increase rapidly thereby causingcurrents I-,, I I I and I to increase rapidly. The rapid increase ofcurrent I could cause an excessively large increase in the frequency ofthe pulses produced by rate generator 15 and trigger generator 14 ofFIG. 1. v

When the frequency of the pulses from trigger generator 14 is too highthe silicon controlled rectifiers of FIG. 1 may be damaged. The reasonfor possible damage rectifiers of FIG. 1 will now be discussed inconnection with the waveform shown in FIG. 4. When the frequency of thepulses from the trigger generator increase the time duration between thepulses decreases. For example, when the frequency increases the relativepositions of the pulses shown in waveform P and waveform R of FIG. 4change so that the time duration between the time t and time t,decreases. The pulse at time t, causes a silicon controlled rectifier 33to fire from time t to the time t; as shown in waveform I The pulse attime in waveform R causes silicon controlled rectifier 34 to fire from atime. to a time t, as shown in waveform I,. If the time differencebetween time t, and time i should decrease so that the first pulse inwaveform R occurs before time t,, the silicon controlled rectifiers 33and 34 would both be conducting at the same time. When siliconcontrolled rectifiers 33 and 34 are both conducting at the same timethere is a low impedance between the terminals 36 and 37 of theswitching regulator shown in FIG. 1. This low impedance will cause ahigh current to flow from terminal 36 through silicon controlledrectifier 33, primary windings 28 and 30 and silicon controlledrectifier 34 to terminal 37. This high current through the siliconcontrolled rectifiers could cause possible damage to silicon controlledrectifiers 33 and 34.

A feedback circuit in the present invention comprising resistors 82,capacitor 83 and a resistor of the intermediate amplifier prevents asudden drop in the voltage V from causing a sudden increase in thecurrent I from the error amplifier. When current I, through resistor 85increases the voltage across resistor 85 increases. This increase involtage across resistor 85 is coupled through capacitor 83 and resistor82 to the input lead 71 of the differential amplifier 73. This increasein voltage from resistor 85 cancels a portion of the decrease in thevoltage V, so that the total change of voltage at the input lead 71 isgreatly reduced. This reduction in the change of voltage on input lead71 causes a reduction in the change of voltage on output lead 74 ofamplifier 73 and causes a reduction in the amount of increase in currentI This slight increase in output current I causes an increase in thefrequency of the pulses applied to the switching regulator but preventsa large increase in frequency which could cause damage to the siliconcontrolled rectifiers in the switching regulator. The feedback circuitreduces the high frequency gain of the error amplifier and insuresincreased stability of the voltage from the switching regulator.

The error amplifier shown in FIG. 2 also provides protection for theswitching regulator during start-up time and when a short circuit or alow impedance load is connected across the output terminals of theswitching regulator. During start-up time and during the time when a lowimpedance load is across terminals 51 and 52 of the switching regulatorof FIG. 1 the voltage V, has a low value. If this low value of voltagedetermines the value of current delivered by the error amplifier theoutput current I would be very large. This large value of current wouldcause the rate generator to develop high frequency pulses which couldcause possible damage to the silicon controlled rectifiers as describedabove. The circuit in FIG. 2 prevents this high current to the rategenerator when V, is low by providing a comparator circuit whichdisables the differential amplifier during the time when the V is verylow. At this time a current for the rate generator is provided by theminimum current circuit 16 which supplies some current to the rategenerator but does not supply sufficient current to cause the frequencyof the rate generator to be excessively high.

When the voltage V, from the switching regulator and at terminal 98 ofcomparator 58 is low transistor 94 is rendered nonconductive. At thistime a positive voltage at the output lead of amplifier 63 in theconstant current source 54 causes the voltage at the base of transistor93 to be more positive than the voltage at the base of transistor 94.The voltage at the base of transistor 93 causes a current I to flow fromthe base to emitter of transistor 93 and throughresistor 95 to ground.Current I through base to emitter of transistor 93 renders transistor 93conductive so that the impedance between the collector and the emitterof transistor 93 is relatively low. This low impedance in transistor 93causes current to flow from output lead 74 of amplifier 73 throughtransistor 93 and resistor 95 to ground. Current through resistor 95provides a voltage drop of the polarity shown across resistor 95. Thevoltage drop across resistor 95 causes the voltage at the emitter oftransistor 94 to be more positive than the voltage at the base so thattransistor 95 is nonconductive. The low impedance in transistor 93causes the voltage on the output lead 74 of the differential amplifier73 to be relatively low. When the voltage at the output lead 74 of thedifferential amplifier is low transistor 77 in the intermediateamplifier is rendered nonconductive. When transistor 77 is renderednonconductive transistor 78 and transistor 87 are rendered nonconductiveso that the current I no longer flows to the rate generator. A current Iflows from terminal 104 in the minimum current circuit 59 throughresistor 105 and diode 107 to the output lead 109 and current outputterminal 110. This current I supplies the current for the rategenerator. Resistor 105 limits the value of current 1 and prevents'thegeneration of high frequency pulses which could cause damage to thesilicon controlled rectifiers in the switching regulator FIG. 1.

When the voltage V, at the output of the switching regulator increasesto near the normal value, the voltage at terminal 98 in comparator 58causes a current I to flow from terminal 98 through resistor 99, frombase to emitter of transistor 94 through resistor 95 to ground. Currentl renders transistor 94 conductive so that a current flows from terminal96 through resistor 97 from collector to emitter of transistor 94through resistor 95 to ground. Current I produces a relatively largevoltage drop of the polarity shown across resistor 95 so that theemitter of transistor 93 is more positive than the base causingtransistor 93 to be rendered nonconductive. When transistor 93 isrendered nonconductive the impedance between the output lead of thedifferential amplifier and ground is high so that the voltage at outputlead 74 of the differential amplifier again causes the current I, torender transistors 77, 78 and 87 conductive and to control the amount ofcurrent supplied to the rate generator.

When transistor 94 in the comparator 58 is rendered conductive thecurrent l produces a voltage drop of the polarity shown across resistor97 so that the voltage at the collector of transistor 94 decreasesthereby causing the voltage at the junction point 106 in the minimumcurrent circuit 59 to decrease. At this same time the transistor 87 isrendered conductive so that the voltage on the output lead 109 increasesto a value of voltage greater than the voltage at junction point 106.This causes the current I through resistor and diode 107 to decrease toa value of zero. Thus, the only current to the rate generator at thistime is current I through transistor 87.

Thus, it can be seen that during normal operations the voltage V, fromthe output of the switching regulator causes the differential amplifier73, the intermediate amplifier 56, and the current amplifier 57 toprovide the current which determines the frequency of the pulsesdeveloped by the rate generator 15 of FIG. 1. When the output voltage Vfrom the switching regulator decreases below a predetermined value thedifferential amplifier 73 is rendered inoperative and the current to therate generator is supplied by the minimum current circuit 59 of FIG. 2.This provides protection for the switching regulator during start-up andshort-circuit conditions in the switching regulator. The feedbackcircuit comprising resistor 82, capacitor 83 and resistor 85 provideprotection for the switching regulator when the output voltage decreasessuddenly.

While the principles of the invention have now been made clear in anillustrative embodiment, there will be many obvious modifications of thestructure, arrangement, proportions, and components, without departingfrom those principles. The appended claims are intended to cover anysuch modifications within the scope of the invention.

What is claimed is:

1. An error amplifier for sensing the voltage at the output terminal ofa switching regulator and for providing a current having a value whichis determined by the voltage from the regulator, said error amplifiercomprising:

a constant voltage source;

a differential amplifier having first and second input leads and anoutput lead;

means for coupling said first input lead of said differential amplifierto said output terminal of said regulator, said second input lead ofsaid differential amplifier being coupled to said source;

a current amplifier having an input lead and an output lead, said inputlead of said current amplifier being coupled to said output lead of saiddifferential amplifier;

a comparator having first and second input leads and first and secondoutput leads, said comparator having impedances at said first and saidsecond output leads which are determined by the value of voltages onsaid first and said second input terminals, said first input lead ofsaid comparator being coupled to said output terminal of said regulator,said second input lead of said comparator being connected to saidsource, said first output lead of said comparator being connected tosaid output lead of said differential amplifier; and

a minimum current circuit having an input lead and an output lead, saidinput lead of said circuit being connected to said second output lead ofsaid comparator, said output lead of said circuit being connected tosaid output lead of said current amplifi- 2. An error amplifier asdefined in claim 1 including:

feedback means, said feedback means being connected between said outputlead of said differential amplifier and said first input lead of saiddifferential amplifier, said feedback means providing a signal to saidfirst input lead of said differential amplifier to prevent suddenchanges in voltage from the switching regulator from causing largechanges in the value of current provided by the error amplifier. v p

3. An error amplifier as defined in claim 1 including:

an intermediate amplifier having an input lead and first and secondoutput leads, said first and said second output leads providing signalshaving opposite phases, said input lead of said intermediate amplifierbeing connected to said output lead of said differential amplifier, saidfirst output lead of said intermediate amplifier being connected to saidinput lead of said current amplifier, said second output lead of saidintermediate amplifier being connected to said first input lead of saiddifferential amplifier.

4. An error amplifier for sensing the voltage at the output terminal ofa switching regulator and for providing a current having a value whichis determined by the voltage from the regulator, said error amplifiercomprising:

a constant voltage source; a differential amplifier having first andsecond input leads and an output lead;

means for coupling said first input lead of said dif- 6 ferentialamplifier to said output terminal of said regulator, said second inputlead of said differential amplifier being coupled to said source;

first and second reference potentials; first, second and thirdtransistors each having a base, a collector and an emitter, saidcollector of said first transistor being connected to said firstpotential, said base of said first transistor being coupled to saidoutput lead of said differential amplifier, said emitter of said firsttransistor being connected to said base of said second transistor;

first, second and third resistors, said first resistor being connectedbetween said first potential and said collector of said secondtransistor, said collector of said second transistor being connected tosaid base of said third transistor, said second resistor being connectedbetween said first potential and said emitter of said third transistor,a first end i of said third resistor being connected to said secondpotential;

a zener diode, said zener diode being connected to said emitter of saidsecond transistor and a second end of said third resistor;

a capacitor, said capacitor being connected between said second end ofsaid third resistor and said first input lead of said differentialamplifier;

a current output terminal, said current output terminal being connectedto said collector of said third transistor;

a comparator having first and second input leads and first and secondoutput leads, said comparator having impedances at said first and saidsecond output leads which are determined by the value of voltages onsaid first and said second input terminals, said first input lead ofsaid comparator being coupled to said output terminal of said regulator,saidsecond input lead of said comparator being connected to said source,said first output lead of said comparator being connected to said baseof said first transistor; and

a minimum current circuit having an input lead and an output lead, saidinput lead of said circuit being connected to said second output lead ofsaid comparator, said output lead of said circuit being connected tosaid current output terminal.

5. An error amplifier as defined in claim 4 wherein said comparatorincludes:

fourth and fifth transistors each having a base, a collector and anemitter, said base of said fourth transistor being connected to saidsecond input lead of said comparator, said base of said fifth transistorbeing connected to said first input lead of said comparator;

a third reference potential;

fourth and fifth resistors, said fourth resistor being connected betweensaid third potential and said collector of said fifth transistor, saidcollector of said fourth transistor being connected to said first outputof the comparator, said second output lead of said comparator beingconnected to said collector of said fifth transistor, said fifthresistor being connected between said second potential and said Iemitters of said fourth and said fifth transistors. 6. An erroramplifier as defined in claim 4 wherein said minimum current circuitcomprises:

first and said second diodes, said cathode of said first diode beingconnected to said second output lead of said comparator, said cathode ofsaid second diode being connected to said current output terminal.

An error amplifier for sensing the voltage at the output terminal of aswitching regulator and for providing a current having a value which isdetermined by the voltage from the regulator, said error amplifiercomprising:

a constant voltage source;

a differential amplifier having first and second input leads and anoutput lead;

means for coupling said first input lead of said differential amplifierto said output terminal of said regulator, said second input lead ofsaid differential amplifier being coupled to said source;

first, second, third, fourth and fifth transistors each having a base, acollector and an emitter, said base of said first transistor beingconnected to said output lead of said differential amplifier, saidemitter of said first transistor being connected to said base of saidsecond transistor, said base of said third transistor being connected tosaid collector of said second transistor;

, first, second and third reference potentials, said collector of saidfirst transistor being connected to said first potential;

first, second, third, fourth, fifth, and sixth resistors,

said regulator, said collector of said fourth transistor being connectedto said base of said first transistor, said fourth resistor beingconnected between said third potential and said collector of said fifthtransistor;

a zener diode, a first end of said third resistor being connected tosaid second potential, said zener diode being connected between a secondend of said third resistor and said emitter of said second transistor,said fifth resistor being connected between said second potential andsaid emitters of said fourth and said fifth transistors;

a current output terminal, said current output terminal being connectedto said collector of said third transistor;

a capacitor, said capacitor being connected between said second end ofsaid third resistor and said first input lead of said differentialamplifier; and

first and second diodes each having an anode and a cathode, said sixthresistor being connected between said first potential and said anodes ofsaid first and said second diodes, said cathode of said first diodebeing connected to said collector of said fifth transistor, said cathodeof said second diode being connected to said collector of said thirdtransistor.

8. An error amplifier as defined in claim 7 including: means for settingthe value of voltage at the output terminal of said switching regulator,said means for setting being connecte between said constant voltagesource and said second input lead of said differential amplifier, saidmeans for setting supplying an adjustable voltage to said second inputlead of said differential amplifier, said adjustable voltage determiningthe average value of current supplied by said error amplifier.

1. An error amplifier for sensing the voltage at the output terminal ofa switching regulator and for providing a current having a value whichis determined by the voltage from the regulator, said error amplifiercomprising: a constant voltage source; a differential amplifier havingfirst and second input leads and an output lead; means for coupling saidfirst input lead of said differential amplifier to said output terminalof said regulator, said second input lead of said differential amplifierbeing coupled to said source; a current amplifier having an input leadand an output lead, said input lead of said current amplifier beingcoupled to said output lead of said differential amplifier; a comparatorhaving first and second input leads and first and second output leads,said comparator having impedances at said first and said second outputleads which are determined by the value of voltages on said first andsaid second input terminals, said first input lead of said comparatorbeing coupled to said output terminal of said regulator, said secondinput lead of said comparator being connected to said source, said firstoutput lead of said comparator being connected to said output lead ofsaid differential amplifier; and a minimum current circuit having aninput lead and an output lead, said input lead of said circuit beingconnected to said second output lead of said comparator, said outputlead of said circuit being connected to said output lead of said currentamplifier.
 2. An error amplifier as defined in claim 1 including:feedback means, said feedback means being connected between said outputlead of said differential amplifier and said first input lead of saiddifferential amplifier, said feedback means providing a signal to saidfirst input lead of said differential amplifier to prevent suddenchanges in voltage from the switching regulator from causing largechanges in the value of current provided by the error amplifier.
 3. Anerror amplifier as defined in claim 1 including: an intermediateamplifier having an input lead and first and second output leads, saidfirst and said second output leads providing signals having oppositephases, said input lead of said intermediate amplifier being connectedto said output lead of said differential amplifier, said first outputlead of said intermediate amplifier being connected to said input leadof said current amplifier, said second output lead of said intermediateamplifier being connected to said first input lead of said differentialamplifier.
 4. An error amplifier for sensing the voltage at the outputterminal of a switching regulator and for providing a current having avalue which is determined by the voltage from the regulator, said erroramplifier comprising: a constant voltage source; a differentialamplifier having first and second input leads and an output lead; meansfor coupling said first input lead of said differential amplifier tosaid output terminal of said regulator, said second input lead of saiddifferential amplifier being coupled to said source; first and secondreference potentials; first, second and third transistors each having abase, a collector and an emitter, said collector of said firsttransistor being connected to said first potential, said base of saidfirst transistor being coupled to said output lead of said differentialamplifier, said emitter of said first transistor being connected to saidbase of said second transistor; first, second and third resistors, saidfirst resistor being connected between said first potential and saidcollector of said second transistor, said collector of said secondtransistor being connected to said base of said third transistor, saidsecond resistor being connected between said first potential and saidemitter of said third transistor, a first end of said third resistorbeing connected to said second potential; a zener diode, said zenerdiode beinG connected to said emitter of said second transistor and asecond end of said third resistor; a capacitor, said capacitor beingconnected between said second end of said third resistor and said firstinput lead of said differential amplifier; a current output terminal,said current output terminal being connected to said collector of saidthird transistor; a comparator having first and second input leads andfirst and second output leads, said comparator having impedances at saidfirst and said second output leads which are determined by the value ofvoltages on said first and said second input terminals, said first inputlead of said comparator being coupled to said output terminal of saidregulator, said second input lead of said comparator being connected tosaid source, said first output lead of said comparator being connectedto said base of said first transistor; and a minimum current circuithaving an input lead and an output lead, said input lead of said circuitbeing connected to said second output lead of said comparator, saidoutput lead of said circuit being connected to said current outputterminal.
 5. An error amplifier as defined in claim 4 wherein saidcomparator includes: fourth and fifth transistors each having a base, acollector and an emitter, said base of said fourth transistor beingconnected to said second input lead of said comparator, said base ofsaid fifth transistor being connected to said first input lead of saidcomparator; a third reference potential; fourth and fifth resistors,said fourth resistor being connected between said third potential andsaid collector of said fifth transistor, said collector of said fourthtransistor being connected to said first output of the comparator, saidsecond output lead of said comparator being connected to said collectorof said fifth transistor, said fifth resistor being connected betweensaid second potential and said emitters of said fourth and said fifthtransistors.
 6. An error amplifier as defined in claim 4 wherein saidminimum current circuit comprises: first and second diodes, each havingan anode and a cathode; and a sixth resistor, said sixth resistor beingconnected between said first potential and said anodes of said first andsaid second diodes, said cathode of said first diode being connected tosaid second output lead of said comparator, said cathode of said seconddiode being connected to said current output terminal.
 7. An erroramplifier for sensing the voltage at the output terminal of a switchingregulator and for providing a current having a value which is determinedby the voltage from the regulator, said error amplifier comprising: aconstant voltage source; a differential amplifier having first andsecond input leads and an output lead; means for coupling said firstinput lead of said differential amplifier to said output terminal ofsaid regulator, said second input lead of said differential amplifierbeing coupled to said source; first, second, third, fourth and fifthtransistors each having a base, a collector and an emitter, said base ofsaid first transistor being connected to said output lead of saiddifferential amplifier, said emitter of said first transistor beingconnected to said base of said second transistor, said base of saidthird transistor being connected to said collector of said secondtransistor; first, second and third reference potentials, said collectorof said first transistor being connected to said first potential; first,second, third, fourth, fifth, and sixth resistors, said first resistorbeing connected between said first potential and said collector of saidsecond transistor, said second resistor being connected between saidfirst potential and said emitter of said third transistor, said base ofsaid fourth transistor being coupled to said source, said base of saidfifth transistor being coupled to said output terminal of saidregulator, said collector of said fourth Transistor being connected tosaid base of said first transistor, said fourth resistor being connectedbetween said third potential and said collector of said fifthtransistor; a zener diode, a first end of said third resistor beingconnected to said second potential, said zener diode being connectedbetween a second end of said third resistor and said emitter of saidsecond transistor, said fifth resistor being connected between saidsecond potential and said emitters of said fourth and said fifthtransistors; a current output terminal, said current output terminalbeing connected to said collector of said third transistor; a capacitor,said capacitor being connected between said second end of said thirdresistor and said first input lead of said differential amplifier; andfirst and second diodes each having an anode and a cathode, said sixthresistor being connected between said first potential and said anodes ofsaid first and said second diodes, said cathode of said first diodebeing connected to said collector of said fifth transistor, said cathodeof said second diode being connected to said collector of said thirdtransistor.
 8. An error amplifier as defined in claim 7 including: meansfor setting the value of voltage at the output terminal of saidswitching regulator, said means for setting being connected between saidconstant voltage source and said second input lead of said differentialamplifier, said means for setting supplying an adjustable voltage tosaid second input lead of said differential amplifier, said adjustablevoltage determining the average value of current supplied by said erroramplifier.